No sabes lo bien que me siento cuando hablo español sin miedo.

Yes, literally it is you don't know, but in this kind of sentence it often works idiomatically as you have no idea.

So:

No sabes lo bien que me siento...
= You have no idea how good I feel...

It is a very natural way in Spanish to emphasise something strongly.

Also, saber is the right verb here because it is about knowing information or a fact. Spanish uses conocer for being familiar with people, places, or things, not for a whole clause like this one.

Lo bien que is a very common Spanish structure used for emphasis. It means something like how well or how good in English, depending on the context.

Pattern:

lo + adjective/adverb + que

Examples:

• No sabes lo difícil que fue. = You don't know how difficult it was.
• Mira lo rápido que corre. = Look how fast he runs.
• No sabes lo bien que me siento. = You don't know how good I feel.

So here, lo bien que me siento means how good I feel.

Because in lo bien que, the word que is part of the fixed structure lo + adjective/adverb + que.

So:

• lo bien que... = how well / how good...

This que does not take an accent.

By contrast, qué with an accent is used in direct and indirect questions or exclamations, for example:

• ¿Qué dices? = What are you saying?
• No sé qué quiere. = I don't know what he wants.

A learner may also see No sabes qué bien me siento, which is possible too, but the sentence you were given uses the very common pattern lo bien que.

Because Spanish normally says sentirse bien for to feel good/well.

So:

• Me siento bien. = I feel good.
• Me siento mal. = I feel bad.

Even though bien is often taught as an adverb, with sentirse this is just the normal idiomatic expression.

Bueno is usually an adjective used with nouns:

• un buen libro
• una persona buena

So me siento bueno would not be the normal way to say I feel good here.

The verb here is sentirse, which means to feel in the sense of to feel a certain way.

So:

• me siento = I feel
• te sientes = you feel
• se siente = he/she feels

The me is the reflexive pronoun that goes with sentirse.

Compare:

• Siento miedo. = I feel fear.
  Here the verb is sentir, and the thing felt is a noun.
• Me siento feliz. = I feel happy.
  Here the verb is sentirse, meaning to be in a certain state.

In your sentence:

• me siento = I feel
• me siento bien = I feel good

Because Spanish often leaves subject pronouns out when the verb ending already shows who the subject is.

In this sentence:

• sabes already tells you the subject is tú
• siento already tells you the subject is yo
• hablo already tells you the subject is yo

So Spanish naturally says:

• No sabes lo bien que me siento cuando hablo español sin miedo.

You could add pronouns for emphasis:

• Tú no sabes lo bien que yo me siento...

But that sounds more marked or contrastive, not neutral.

Because this sentence talks about something habitual or real: when I speak Spanish without fear.

So Spanish uses the indicative:

• cuando hablo español sin miedo = when I speak Spanish without fear

The speaker means this is something that happens, not just a possible future event.

Compare:

• Cuando hablo español sin miedo, me siento muy bien.
  Habitual/real → indicative
• Cuando hable español sin miedo, me sentiré mejor.
  Future/not yet realised → subjunctive

So in your sentence, hablo is correct because it refers to a real repeated situation.

After verbs like hablar, aprender, enseñar, and estudiar, Spanish usually does not use the article before the name of a language.

So:

• Hablo español.
• Aprendo francés.
• Enseña inglés.

But when the language is the subject of the sentence, Spanish often does use the article:

• El español es difícil.
• El inglés se habla en muchos países.

So here, hablo español is the normal form.

Because in Spanish, names of languages are normally written with a lowercase letter, unlike in English.

So Spanish writes:

• español
• inglés
• francés

English writes:

• Spanish
• English
• French

This is a normal spelling difference between the two languages.

Because sin miedo means without fear in a general sense. Spanish usually leaves out the article with abstract nouns in this kind of expression.

So:

• sin miedo = without fear
• sin problemas = without problems
• sin duda = without doubt

If you say sin el miedo, you are usually referring to a specific fear already mentioned:

• sin el miedo de antes = without the fear from before

In your sentence, the meaning is general, so sin miedo is the natural choice.

Not very naturally. A word-for-word version would be something like:

You don't know how well I feel when I speak Spanish without fear.

English usually prefers:

• You have no idea how good I feel when I speak Spanish without fear.
• You don't know how good I feel when I speak Spanish without fear.

So the Spanish structure is clear, but the most natural English translation often needs a small adjustment, especially with No sabes... and me siento bien.